In many technical systems, digital signals are converted into analog signals using pulse width modulations. Examples of such systems are engine controllers, calibration circuits, switched mode power supplies, DC voltage converters, digital amplifiers and, in particular, digital audio amplifiers.
In the aforementioned systems, the digital signal is normally available on the input side in the form of a PCM (pulse code modulated) signal and is first of all converted into a digital PWM (pulse width modulated) signal. Next, the digital PWM signal is converted into an analog signal. Both the PCM/PWM conversion and the conversion of the PWM signal into the analog signal may experience nonlinear distortions. However, PCM/PWM conversion can be performed without losses, in principle, on account of the digital signal processing, i.e. the original PCM signal could be generated from the PWM signal by an inverted conversion without loss of information. In contrast, nonlinear distortions inevitably arise when converting the digital PWM signal into the analog signal. These distortions are caused primarily by the “memory” of the analog area of the circuit in question. The relatively long decay times for the analog “memory”, which normally go beyond the length of time of a PWM period, cause decaying square-wave responses from the past to be overlayed with the current square-wave response. Since the square-wave responses from the past are dependent on the pulse width of the digital PWM signal, and these are in turn dependent on the PCM signal, the distortions are nonlinear.
Such “memory” properties of an analog circuit are system inherent and cannot be eliminated completely. For this reason, ways are sought to minimize or even to compensate for the nonlinear distortions arising in the digital/analog conversion through additional digital or analog circuits.
The international patent application WO 97/37433 A1, representing the closest prior art, describes a digital/analog converter circuit which operates according to the above principle and in which the signal path contains a nonlinear Hammerstein filter upstream of the PCM/PWM converter stage. The Hammerstein filter is a precompensation stage which subjects the PCM signal to nonlinear distortion which is the inverse of the nonlinear distortion arising during the PCM/PWM conversion, which means that a distortion free digital PWM signal is obtained overall. A drawback of this digital/analog converter circuit is that it is designed exclusively for UPWM (uniform sampling pulse width modulated) signals. In the case of UPWM signals, the value of the pulse width is always stipulated at the start of the PWM period. PWM signals whose pulse width is determined during the PWM period, so-called PNPWM (pseudo natural pulse width modulation) signals, cannot be generated using the present circuit.
The international patent application WO 95/06980 A1 demonstrates another way of compensating for the nonlinearities which arise during PCM/PWM conversion. To this end, a digital/analog converter circuit has access to a table which is stored in a read only memory. The data stored in the table are used to compensate for the nonlinear distortions.
A relatively complex algorithm for digital compensation for nonlinear distortions by a noise shaper connected upstream of the PCM/PWM converter stage using a specially aligned digital filter in the feedback path is described in the article “New high accuracy pulse width modulation based digital-to-analogue converter/power amplifier” by J. M. Goldberg and M. B. Sandler, which appeared in IEE Proc.-Circuits Devices Syst., volume 141, No. 4, 1994, on pages 315-324.
In addition, analog circuits are known, which minimize nonlinearities via feedback paths. For example the articles “A New PWM Controller with One-Cycle Response” by K. M. Smith, Z. Lai and K. M. Smedley, which appeared in IEEE Transactions on Power Electronics, volume 14, No. 1, 1999, on pages 142-150, and “A Novel Low-power Low-voltage Class D Amplifier with Feedback for Improving THD, Power Efficiency and Gain Linearity” by J. S. Chang, B. H. Gwee, Y. S. Lon and M. T. Tan, which appeared in ISACS 2001, volume 1, on pages 635-638, are concerned with such analog feedback circuits. A drawback of feedback circuits is the instabilities they cause.